The present invention generally relates to a liquid crystal switching device and, in particular, relates to one such liquid crystal switching device including means, disposed at the Brewster angle to the incident light beam, for splitting the incident light beam into the polarized components thereof.
Liquid crystal switching devices, per se, have been known for some time. Typical of such switching devices are those described and discussed in U.S. Pat. No. 4,201,422 issued to McMahon et al. on May 6, 1980, U.S. Pat. No. 4,278,327 issued to McMahon et al. on July 14, 1981, and U.S. Pat. No. 4,385,799 issued to Soref on May 31, 1983. Therein a plurality of liquid crystal switch designs are described wherein an inputted light beam is incident upon a layer of liquid crystal material at the critical angle thereof. In addition, these basic designs include optical fibers attached to the side angled surfaces of a pair of opposing trapezoidal prisms. The trapezoidal prisms are arranged with the bases thereof parallel and include liquid crystal material disposed therebetween.
The device designs so described are both expensive and difficult to manufacture since all of the surfaces of the trapezoidal prisms must not only be optically flat, but must also be fixed at a precise angular relation, specifically the critical angle, to each other.
Consequently, the liquid crystal optical switches described in these U.S. Patents are rather impractical for optical communication systems wherein large numbers of such devices are needed.
More recently a number of inexpensive, readily mass produceable liquid crystal device designs have been described and discussed in the U.S. patent application Ser. Nos. 795,138; 795,148; 795,149; 795,150; 795,151; 795,152; 795,154; 795,155; 795,156; 795,157; 795,296 all filed on Nov. 5, 1985 and assigned to the assignee hereof. These applications are incorporated herein by reference.
In general, all of the liquid crystal switching devices shown and described to date can be characterized as either critical angle devices or evanescent wave coupling devices. In the evanescent wave coupling devices the cores of adjacently disposed optical fibers are exposed such that light signals are coupled across the gap therebetween. In the remainder of the devices, a light beam must be introduced into the device at the critical angle.
As well known in the field of optics, the critical angle is that minimum angle measured from normal incidence whereat an incident light beam is totally reflected at an interface between materials having different indicies of refraction. The critical angle is dependent upon the indicies of refraction of the materials defining the interface whereat the light beam is to be split. In most instances, in the devices described above, the interface includes a layer of liquid crystal material.
As it happens, liquid crystal material is, essentially, a birefringent material, that is, the material exhibits a different index of refraction for each of two different plane polarizations. Using this phenomena, an incident light beam is made to impinge upon a layer of liquid crystal material at an angle such that only one of the plane polarized light components is, effectively, totally reflected. Thus, an incident light beam is divided into the plane polarizations thereof since one plane polarized component is reflected and the other plane polarized component is transmitted through the liquid crystal material.
Thus, typical device designs that have been proposed using liquid crystal material not only to effect beam splitting and beam recombining but also to controllably reorient the polarization of the polarized components. In most of these designs the incident light beam is directed at a layer of liquid crystal material at the critical angle. As a result, the overall length of such devices are geometrically dependent upon the critical angle. Depending on the relevant indexes of refraction the critical angle can become rather large. As a result the overall length of such devices can also be large. This can become a factor in the design of a switching matrix wherein a considerable number of such switching devices are oriented end-to-end.
Consequently, a liquid crystal switching device that does not rely on the comparatively large critical angle and can thus be made shorter is needed to more fully expand the potential benefits of liquid crystal devices and hence, optical signalling systems.